1. Field of the Invention
The present invention relates to thickness-extensional piezoelectric resonators which utilize harmonics in a thickness extensional vibration mode, and more particularly, to a thickness-extensional piezoelectric resonator which suppresses spurious resonances caused by a fundamental wave, via an improved electrode shape.
2. Description of the Related Art
As a MHz-band piezoelectric resonator used for generating a clock signal for a microcomputer, for example, a piezoelectric resonator utilizing a harmonic in a thickness-extensional vibration mode is known. Since such a piezoelectric resonator uses a harmonic in a thickness-extensional vibration, a response caused by the fundamental wave in the thickness-extensional vibration is a spurious resonance. Therefore, it is urgently needed to suppress a spurious resonance caused by the fundamental wave.
In Japanese Unexamined Patent Publication No. 4-216208, a piezoelectric resonator is disclosed which uses a third harmonic in the thickness-extensional vibration and which can suppress, via the use of floating electrodes, a spurious resonance caused by the fundamental wave.
FIG. 15 shows a structure of the piezoelectric resonator described in the above patent publication. In a piezoelectric resonator 61, a vibration electrode 63 is located at the center of the first major surface of a rectangular piezoelectric substrate 62 and a vibration electrode 64 is provided at the center of the second major surface. The vibration electrodes 63 and 64 are opposed to each other with the piezoelectric substrate 62 located therebetween.
The vibration electrode 63 is connected to a terminal electrode 65b provided along an edge at a short side of the piezoelectric substrate 62, through a lead electrode 65a. The vibration electrode 64 is electrically connected to a terminal electrode 66b provided along an edge at a short side, via a lead electrode 66a provided on the second major surface of the piezoelectric substrate 62.
On the first major surface of the piezoelectric substrate 62, floating electrodes 67a and 67b are provided along edges at a pair of long sides of the piezoelectric substrate 62, and floating electrodes 67c and 67d are provided on the second major surface such that they are opposed to the floating electrodes 67a and 67b with the piezoelectric substrate 62 located therebetween.
In the piezoelectric resonator 61, a portion where the vibration electrodes 63 and 64 are opposed to each other defines a vibration section, and the third harmonic of the thickness-extensional vibration is trapped in the vibration section. The fundamental wave is transferred from the vibration section to a surrounding area. Due to mechanical loads and piezoelectric short-circuit effects of the floating electrodes 67a to 67d, the portions where the floating electrodes 67a to 67d are located absorb the vibration energy of the fundamental wave, and an unwanted spurious resonance caused by the fundamental wave is suppressed.
Since the floating electrodes 67a to 67d need to be provided at both sides of the vibration section and extend in the short-side direction of the piezoelectric substrate 62 in the piezoelectric resonator 61, however, the piezoelectric resonator 61 cannot be made compact due to the portions where the floating electrodes 67a to 67d are located. Since the areas of the floating electrodes 67a to 67d must be large in order to guide the fundamental wave to the portions where the floating electrodes 67a to 67d are located, it is very difficult to suppress a spurious resonance caused by the fundamental wave and to make the piezoelectric resonator 61 compact at the same time.
In order to overcome the problems described above, preferred embodiments of the present invention provide a thickness-extensional piezoelectric resonator utilizing a harmonic in a thickness-extensional vibration mode, which can effectively suppress an unwanted spurious resonance caused by the fundamental wave and which can also be made compact.
A preferred embodiment of the present invention provides a piezoelectric resonator utilizing a harmonic in a thickness-extensional vibration mode, including a piezoelectric substrate, first and second excitation electrodes provided partially on a first major surface and a second major surface of the piezoelectric substrate, respectively such that they are opposed to each other via the piezoelectric substrate, first and second lead electrodes connected to the first and second excitation electrodes and extending toward edges of the piezoelectric substrate, respectively, first and second terminal electrodes connected to the first and second lead electrodes and provided along edges of the piezoelectric substrate, respectively, and a spurious suppressing electrode section connected to a portion of at least one of the first and second lead electrodes.
Since the spurious suppressing electrode section is connected to a portion of at least one of the first and second lead electrodes, which are connected to the first and second excitation electrodes, in the thickness-extensional piezoelectric resonator according to one preferred embodiment of the present invention, the fundamental wave in the thickness-extensional vibration mode is effectively led towards ends of the piezoelectric substrate by the first and second lead electrodes. Therefore, by securing the thickness-extensional piezoelectric resonator with the use of the first and second terminal electrodes connected to the first and second lead electrodes, the led fundamental wave is effectively damped, and thereby a spurious resonance caused by the fundamental wave is suppressed. Consequently, a thickness-extensional piezoelectric resonator using a harmonic in the thickness-extensional vibration mode and having excellent resonance characteristics is provided.
In contrast to the conventional thickness-extensional piezoelectric resonator which is prevented from being made compact when the fundamental wave is suppressed by the floating electrodes since it is required in the conventional thickness-extensional piezoelectric resonator that the floating electrodes be provided at both sides of the vibration section in the short-side direction of the piezoelectric substrate, the piezoelectric resonator according to preferred embodiments of the present invention is not prevented from being made compact since the spurious suppressing electrode section is connected to a portion of the lead electrodes.
In the above described piezoelectric resonator, the spurious suppressing electrode section may be arranged to extend in a direction which intersects with the direction in which the lead electrodes extend, so as to increase the width of the lead electrodes.
In this case, since the spurious suppressing electrode section is arranged to extend in a direction which intersects with the direction in which the lead electrodes extend so as to increase the width of the lead electrodes, the spurious suppressing electrode section can be easily provided in a process in which the lead electrodes are provided. In addition, the spurious suppressing electrode section can be easily provided just by increasing the width of the lead electrodes when the lead electrodes are formed. Further, even though a spurious resonance caused by the fundamental wave is suppressed, the piezoelectric resonator can still be made compact.
In the above described piezoelectric resonator, the width dimension of a portion of the lead electrodes where the spurious suppressing electrode section is provided may be equal to or more than about a half of the width dimension of the excitation electrodes.
It is preferred that the width dimension of a portion of the lead electrodes where the spurious suppressing electrode section is provided be equal to or greater than the width dimension of the excitation electrodes.
The thickness-extensional piezoelectric resonator may be configured such that the first excitation electrode has a substantially circular shape provided substantially at the center on the first major surface of the piezoelectric substrate; one substantially rectangular electrode film, including a region opposed to the first excitation electrode via the piezoelectric substrate, defines the second excitation electrode, the second lead electrode, and the second terminal electrode; and the region opposed to the first excitation electrode via the piezoelectric substrate in the one substantially rectangular electrode film functions as the second excitation electrode.
In this case, since the first excitation electrode has a substantially circular shape provided substantially at the center of the first major surface of the piezoelectric substrate; one substantially rectangular electrode film, including a region opposed to the first excitation electrode via the piezoelectric substrate, defines the second excitation electrode, the second lead electrode, and the second terminal electrode; and the region opposed to the first excitation electrode via the piezoelectric substrate, and the one substantially rectangular electrode film functions as the second excitation electrode section, a spurious resonance caused by the fundamental wave is suppressed by the spurious suppressing electrode provided at sides of the second lead electrode in the substantially rectangular electrode film, and variations in the overlapping area between the first and second excitation electrodes are significantly reduced.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.